If you run a system that hosts multiple users, you might want to discourage them from using cron or at for whatever reasons. This can be done.
To restrict people from using cron, create a file called /etc/cron.deny and put the name of the user you want to restrict in there.
To restrict people from using at, create a file called/etc/at.deny, and put the name of the user you want to restrict in there. Be careful about restricting default system users like nobody. These accounts sometimes run their own jobs at specific times.